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package org.apache.juneau.http;

/**
 * Represents a parsed <l>Vary</l> HTTP response header.
 * 
 * <p>
 * Tells downstream proxies how to match future request headers to decide whether the cached response can be used rather
 * than requesting a fresh one from the origin server.
 * 
 * <h5 class='figure'>Example</h5>
 * <p class='bcode'>
 * 	Vary: *
 * 	Vary: Accept-Language
 * </p>
 * 
 * <h5 class='topic'>RFC2616 Specification</h5>
 * 
 * The Vary field value indicates the set of request-header fields that fully determines, while the response is fresh,
 * whether a cache is permitted to use the response to reply to a subsequent request without revalidation.
 * For uncacheable or stale responses, the Vary field value advises the user agent about the criteria that were used to
 * select the representation.
 * A Vary field value of "*" implies that a cache cannot determine from the request headers of a subsequent request
 * whether this response is the appropriate representation.
 * See section 13.6 for use of the Vary header field by caches.
 * <p class='bcode'>
 * 	Vary  = "Vary" ":" ( "*" | 1#field-name )
 * </p>
 * 
 * <p>
 * An HTTP/1.1 server SHOULD include a Vary header field with any cacheable response that is subject to server-driven
 * negotiation.
 * Doing so allows a cache to properly interpret future requests on that resource and informs the user agent about the
 * presence of negotiation on that resource.
 * A server MAY include a Vary header field with a non-cacheable response that is subject to server-driven negotiation,
 * since this might provide the user agent with useful information about the dimensions over which the response varies
 * at the time of the response.
 * 
 * <p>
 * A Vary field value consisting of a list of field-names signals that the representation selected for the response is
 * based on a selection algorithm which considers ONLY the listed request-header field values in selecting the most
 * appropriate representation.
 * A cache MAY assume that the same selection will be made for future requests with the same values for the listed
 * field names, for the duration of time for which the response is fresh.
 * 
 * <p>
 * The field-names given are not limited to the set of standard request-header fields defined by this specification.
 * Field names are case-insensitive.
 * 
 * <p>
 * A Vary field value of "*" signals that unspecified parameters not limited to the request-headers (e.g., the network
 * address of the client), play a role in the selection of the response representation.
 * The "*" value MUST NOT be generated by a proxy server; it may only be generated by an origin server.
 * 
 * <h5 class='section'>See Also:</h5>
 * <ul class='doctree'>
 * 	<li class='extlink'><a class='doclink' href='https://www.w3.org/Protocols/rfc2616/rfc2616.html'>Hypertext Transfer Protocol -- HTTP/1.1</a>
 * </ul>
 */
public final class Vary extends HeaderString {

	/**
	 * Returns a parsed <code>Vary</code> header.
	 * 
	 * @param value The <code>Vary</code> header string.
	 * @return The parsed <code>Vary</code> header, or <jk>null</jk> if the string was null.
	 */
	public static Vary forString(String value) {
		if (value == null)
			return null;
		return new Vary(value);
	}

	private Vary(String value) {
		super(value);
	}
}
